This invention relates generally to computed tomographs commonly referred to as CT's for nondestructively obtaining a tomographic information of an object being examined by employing radiation, and more particularly to a device for holding an object to be examined in computed tomographs.
X-ray CT's are commonly used for nondestructively obtaining a tomographic information of an object to be examined. There has been proposed a CT of such a type that obtains a tomographic information of an object by irradiating the object with light in the visible and near infrared regions as disclosed in, for example, Japanese Unexamined Patent Publication No. 60-72542.
The apparatus generally includes a light beam scanning device which comprises scanning means for causing the light beam from a source to transmit through different parts of an object to be examined, and a photodetector opposite the scanning means to receive the light emitted by the scanning means and transmitted through the object.
The disclosed CT is schematically shown in FIGS. 5 and 6. FIG. 5 is a side elevational view, partly broken away, of the apparatus and FIG. 6 is a sectional view taken along line A--A in FIG. 5.
The apparatus comprises a gantry 1 in the form of a cylindrical drum, which is rotatably supported at the outer circumferential surface 1c thereof by means of rollers 2. An object 3 to be examined is placed adjacent the axis X of the gantry 1.
The gantry is formed on the entire circumferential surface 1c thereof with a screw groove, not shown, which is engaged by a gear not shown but driven by a motor to rotate the gantry about its axis X.
Inside the gantry there are provided a light source 5 for emitting a beam 6 of light, a reflector 7 for directing the light beam 6 onto the object 3 to be scanned and a scanner 8 for swinging the reflector 7 so that the beam 6 scans a sector-shaped range through an angle .theta. sufficient to cover the area of the object 3 to be examined.
The light source 5, the reflector 7 and the scanner 8 constitute the previously mentioned beam scanning means.
Inside the gantry, there is also provided a photodetector 9 at the side of the object opposite the reflector 7 and at such a position as to be able to receive the light beam 6 that has scanned the object 3. The photodetector 9 comprises a plurality of photosensitive elements arranged along an arc of a circle whose center coincides with a point at the surface of the reflector 7. The photodetector can be a photodiode array or a plurality of optical fibers each having one end arranged along the above-mentioned arc and the opposite end connected to a photomultiplier tube not shown.
With an object 3 to be examined being set in place and the gantry 1 kept stationary, the reflector 7 is swung so as to cause the light beam 6 to scan the sector-shaped range of the angle .theta.. Then the gantry 1 is rotated a predetermined angle and kept stationary and the light beam 6 is caused to scan in a similar manner. By repeating the above-mentioned rotation of the gantry 1 and scanning of the object by the light beam 6 it is possible to successively change the direction of incidence of the light beam on an object being examined at predetermined intervals circumferentially of the object and scan the object in all directions through 360.degree..
Inside the gantry 1 the object 3 is held by a holder, a typical example of which is shown in FIG. 7 in the form of a hollow cylinder 4 made of transparent glass. The beam 6 scans in a plane (to be referred to as the scanning plane) perpendicular to the axis of the cylindrical holder 4 as shown in FIG. 8, that is, in parallel with the plane of the drawing sheet. So long as the scanning light beam 6 passes adjacent the center of the cylinder of the holder 4, there is no problem. As the light beam swings toward the periphery of the cylinder, the incident angle .alpha. of the light beam on the cylinder comes to exceed a certain angle, whereupon the light beam cannot pass through the cylinder wall but is reflected as shown at 6a in FIG. 8, so that measurement becomes impossible.
If the diameter of the holder 4 is made of a larger size than the sector-shaped area scanned by the light beam as shown in FIG. 9, the position of the object 3 would be lowered out of the scanning area as the light source is moved about the holder 4, with resulting impossibility of measurement.